AI not colonizing/expanding

Description

I have set up the game so that you can only colonize breathable planets. However, the AI no longer colonizes at all.

I have located the functions that determine what ship a planet will build, but cannot determine why it always skips over the colony ships. If I allow colonizing of non-breathable planets, then the AI goes wild and colonizes everything, breathable and non-breathable atmospheres.

I managed to track down where the AI creates a list of planets to colonise, but it doesn't sort on breathable planets (file: Script_AI_Lists.txt, line 199: "call Sys_Add_Planet_Sort_Column(SORT_PLANET_BY_BREATHABLE, FALSE)").